Air trimmer capacitors are often subjected to thermal cycling and mechanical vibration which can alter the spacing of the capacitor plates, or electrodes as the plates are referred to herein, thereby altering the capacitance of the device itself. To inhibit this from occuring such capacitors have been constructed with a helically threaded engagement between a rotor, that carries a movable electrode, and the capacitor housing which supports or functions as a stationary electrode. The movable electrode is electrically connected with a device terminal through the rotor, and the threads between the rotor and a section of the housing. Since helical threads inherently provide rather poor electrical contacts it has been a common practice to distort the rotor threads so as to produce an engagement mismatch between those of the rotor and housing in order to improve the mechanical contact and electrical coupling therebetween. Such thread distortion also helps to increase the friction or torque between the rotor and housing to minimize frequency drift attributable to vibration-induced migration of the rotor.
Ultra-high frequency circuits are typically quite sensitive to changes in current path geometries. Consequently, threads distorted in air trimmer capacitors provide a current path that may be varied, depending upon the position of the thread distortion with respect to the rotor terminal. When the capacitor is adjusted to provide minimum capacitance, i.e. maximum spacing between electrodes, the thread distortion that provides a current path may be located some distance away from the rotor terminal. Consequently, current path changes occur with tuning which change self-inductance, the Q factor, and the self-resonant frequency. This provides a complication for circuit designers who therefore must treat the capacitor as a variable impedance device. In addition, distorted threads wear at a relatively high rate which produces progressively deteriorating electrical contact and progressively looser rotor fit which renders the device increasingly subject to rotor migration.
In U.S. Pat. No. 4,415,949 which is assigned to the assignee of the present application, an air trimmer capacitor is disclosed which ameliorates these problems by providing a UHF current path which is substantially independent of the position of the movable electrode and rotor, and anti-rotational torque that does not tend to vary as a function of the wear of rotor housing threads. This is accomplished with the provision of a tightly wound coil spring confined within the housing in engagement with rotor threads with adjacent turns of the coil having their centers offset from one another. When the threads of the rotor engage the tightly wound coil spring the eccentric turns of the spring are forced towards relative concentricity because the minor diameter of the rotor threads is greater than the relaxed diameter of the spring. Beyond the threads the spring is brought by a smooth, cylindrical outer surface of the rotor into nearly linear contact with the housing interior wall. Good contact is therefore established all along the wall including that location closest to the rotor terminal whereby the current path is essentially made independent of rotor position and mismatching thread design is avoided.
Though the just-described air trimmer capacitor has provided decided improvements in the art, it has not totally eliminated the previous-described problems. This is at least in part due to the fact that the interior wall of the housing in which the rotor and spring are located is not exactly cylindrical. It rather has a slight conical taper so that it may be die cast rather than machined for economy of manufacture. Were the interior wall to be precisely cylindrical the part could not be easily removed from a die following a casting operation since a void would tend to be created occupied by a partial vacuum as part removal were attempted. Though the conical taper is slight, typically about 1.degree., it nevertheless is sufficient to provide a looseness of fit and contact between the tightly wound coil spring and the interior wall of the housing, even though the adjacent coils of the spring are eccentric. Conceivably it would be possible to overcome this problem by the use of an eccentric spring that itself had an overall conical taper. However, the cost of manufacturing such a spring in a reliably repeatable manner renders such an approach not feasible. Accordingly, it is to the provision of an alternate solution to such problem that the present invention is primarily directed.